The size of magnetic heads has reduced over time with the aim of increasing a recording density of the magnetic disk in hard disk drives (HDDs). Induction heads which employ a combined recording head and read head typically were widespread in the industry, but a more current trend is toward separating the recording head and the read head in a separate recording/read head design in order to improve performance of the head. The recording head may be an induction head for writing information using an induced magnetic field produced by a coil, while the read head used may be a giant magnetoresistive (GMR) head or a tunnel magnetoresistive (TMR) head.
FIG. 2 shows a schematic of a separate recording/playback-type head comprising a recording head part 1 and a read head part 2. Furthermore, FIG. 3 shows an enlarged view of the read head portion. FIG. 3 is a view seen from the surface facing the magnetic disk medium also known as the air bearing surface (ABS), and corresponds to a view seen from A in FIG. 2. TW is the dimension in the track width direction of the read head part 2. The magnetic head is formed on a substrate, possibly comprising alumina-titanium carbide (Al2O3—TiC, referred to below as AlTiC) which may be produced using microfabrication technology. It should be noted that the substrate portion has been omitted in the figures, since it is a consistent feature which does not change during processing of the magnetic head. A lower magnetic shield layer 3 and an upper magnetic shield layer 4 possibly comprising permalloy both play a dual role as electrodes, and magnetism is detected when a multilayer spin valve-type magnetoresistive film 5 formed therebetween is energized. An insulating film 11, possibly comprising alumina, may be used at the portion where the magnetoresistive film 5 is isolated, and a hard bias film 6 for applying a bias field may be disposed adjacent thereto.
Various new technologies for magnetic heads have been introduced that attempt to provide an improvement to the recording density of HDDs. One such technology is the differential read head structure shown in FIG. 4, which was proposed with the aim of improving the magnetic field detection performance (resolution) in the bit length direction of the read head. The differential read head has a configuration in which two spin-valve elements are connected in series, and the respective spin valves change the parity of the total number of exchange-couplings in the fixing layer in such a way that the polarity of the magnetic resistance is reversed. As a result, the polarity of the output of the two elements connected in series is reversed, and the differential signals shown schematically in FIG. 4 are output. The detection sensitivity for the magnetic transition of a medium increases the narrower the magnetic shield gap (GS in FIG. 3) with a normal spin valve. It is difficult to shorten this gap, particularly when it is less than the thickness of the spin valve (less than or equal to about 25 nm), but in the case of a differential read head, it is known that the distance between two magnetic detection layers (GL in FIG. 4) is a governing factor, and that it is possible to improve resolution because this distance can be freely shortened, as described in detail in Japanese Patent No. 3760095.
The structure shown here constitutes a dual element-type differential head structure in which spin valves of the same type are arranged in series, but an AFC-type structure (antiferro coupling, as described in detail in Japanese Unexamined Patent Appl. No. 2009-26400) in which the magnetic detection layers are antiferromagnetically coupled, and a differential structure in which the two elements are connected in parallel has also been considered. In either case, at least two layers are needed for the magnetic detection layer, compared with a normal single-layer spin valve, and therefore the magnetic layer becomes thicker. Due to these factors, the difficulty of processing the dual element-type differential head increases, which constitutes a major difference in production between this type of head and more conventional heads.
Another technology that attempts to increase the recording density of HDDs is the microwave-assisted magnetic recording (MAMR) element, shown in FIG. 5, which was proposed in order to improve the performance of the recording head. The MAMR element has a similar structure to a read head, as disclosed in Xiaochun Zhu and Jian-Gang Zhu, Bias Field Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current, IEEE trans. Magn., 42, No. 10 (2006). A spin polarization current is created in the magnetic layers of the reference layer 20 in FIG. 5, spin torque is used to cause oscillation in a microwave generation layer 19 (FGL, field generation layer), and high-frequency electromagnetic waves (microwaves) may be produced. An auxiliary layer 18, etc., may be provided, depending on the configuration.
One next-generation technology for high recording density involves local irradiation of microwaves, and induction of magnetization precession in the medium so that it is possible to assist in the magnetization inversion in the recording head field, and therefore it is possible to compensate for minute insufficiencies in the recording head field, while it is also possible to make use of a medium material with strong heat fluctuation resistance. The structure of the MAMR element comprises multilayers of magnetic film, as shown in FIG. 5, and it has been calculated that the microwave generation layer 19 should be at least 25 nm in order to generate a high-frequency magnetic field of the desired intensity, and the process for making a thick magnetic film is a key point in terms of production, in the same way as for the differential read head described above.